The present invention relates to incandescent lamps, and more particularly to incandescent lamps made from photonically engineered thermal emitters.
Incandescent lamps offer very high quality lighting, are inexpensive, and are the most popular lighting technology for residential use. They are also, unfortunately, the least efficient (energy to useful light) lighting technology used commercially today. An excellent overview of incandescent lamp technology is given in Bergman et al., Filament Lamps, GE Research and Development Center, Report 98CRD027, February 1998.
The lighting industry commonly uses the term luminous efficacy to describe the efficiency of a lamp. Luminous efficacy is frequently defined as the luminous flux divided by the total radiant power in units of lumens/Watt. The luminous flux has units of lumens, and is the radiant flux weighted by the human eye response. A better description for the efficiency of a lamp is to divide the luminous flux by the total input power to the lighting source, so that the electrical performance can be factored into the comparison of lighting technologies. This disclosure will use the latter definition for luminous efficacy, since some lighting approaches have inherently less efficiency in converting input electrical power into radiant power.
The luminous efficacy of a 60 W incandescent lamp using a tungsten filament is only about 15 lumens/Watt. The luminous efficacy of the incandescent lamp is low because much of the light (around 90%) is emitted by the tungsten filament in the non-visible infrared (wavelengths longer than 760 nm) portion of the spectrum. Fluorescent lamps are much more efficient than incandescent lamps, and have luminous efficacies between 75 and 100 lumens/Watt. By comparison, the theoretical maximum luminous efficacy for high-quality white lighting using a broad spectral source is around 200 lumens/Watt.
An incandescent lamp works by heating up a tungsten filament to a sufficiently high temperature (typically around 2800° K) that it radiates in the visible portion of the electromagnetic spectrum (roughly 380 to 760 nm). Such high-temperature bodies are commonly referred to as “emitters” or “radiators”. The radiation from a high-temperature emitter is described by the theory of blackbodies. An ideal blackbody emits the theoretically maximum radiation. Real emitters do not radiate as well as an ideal blackbody. The emissivity is the ratio of the radiation from a real emitter to the radiation of an ideal blackbody, and is unitless with a value between 0 and 1.
The luminous efficacy of the incandescent lamp can be improved by modifying the emissivity of the emitter. The optimum emitter for lighting purposes would have an emissivity of unity in the visible portion of the spectrum and an emissivity of zero in the non-visible portions of the spectrum. Such an emitter would emit all the light in the useful visible portion of the spectrum and no light in the non-useful non-visible portion of the spectrum. A 2800° K emitter with such an optimized selective emission would have a luminous efficacy approaching 200 lumens/Watt, or over 10× improvement compared to current incandescent lamps and 2× improvement compared to current fluorescent lamps.
There remains a need for a high-temperature emitter that selectively emits radiation in the visible portion of the spectrum, thereby enabling an incandescent lamp having improved luminous efficacy.